Injection needles for medical use generally have an edge surface with two slant angles as viewed in side elevation, which is generally called a lancet point. FIG. 7 is a plan view of the edge surface of a lancet point structure of a conventional injection needle. In FIG. 7, it is assumed that the injection needle has a needle point at a distal end thereof and an opposite end at a proximal end thereof. The injection needle 11 has a needle tube 12 including an edge surface 13 disposed on the distal end. The edge surface 13 includes a first ground facet 13a formed closer to the proximal end, and a second ground facet 13b and a third ground facet 13c that are formed closer to the distal end than the first ground facet 13a. The second ground facet 13b and the third ground facet 13c are symmetrical in shape with respect to the major axis of the edge surface that is of a substantially elliptical shape. The sharp needle point 15 on the distal end of the edge surface 13 is present on a central plane 16 that crosses the first ground facet 13a perpendicularly and includes the central axis of the needle tube 12.
FIGS. 8(a) through 8(c) are views illustrative of a process of forming the edge surface 13 of the lancet point structure, and show in side elevation the injection needle 11 in the vicinity of its distal end illustrated in FIG. 7. For forming the edge surface 13 of the lancet point structure, as shown in FIG. 8(a), a grinding wheel is applied to the distal end portion of the needle tube 12, which is in the form of a hollow tube, at an angle α (0 °<α<90°) with respect to the central axis 18 of the needle tube 12, and grinds the distal end portion to form the first ground facet 13a whose angle with respect to the central axis 18 of the needle tube 12 is α.
Then, as shown in FIG. 8(b), the needle tube 12 is turned a certain angle about the central axis 18 toward the viewer of FIG. 8(b). The grinding wheel is applied to the distal end portion of the needle tube 12 at an angle φ (0°<α<φ<90°) with respect to the first ground facet 13a around the central axis 18 of the needle tube 12, and grinds the distal end portion to form the second ground facet 13b whose angle with respect to the central axis 18 of the needle tube 12 is φ.
Thereafter, as shown in FIG. 8(c), the needle tube 12 is turned about the central axis 18 from the state shown in FIG. 8(a) away from the viewer of FIG. 8(b). The grinding wheel is applied to the distal end portion of the needle tube 12 at an angle θ (0°<α<θ=φ<90°), which is equal to the angle φ, with respect to the first ground facet 13a around the central axis 18 of the needle tube 12, and grinds the distal end portion to form the third ground facet 13c whose angle with respect to the central axis 18 of the needle tube 12 is θ (=φ). The angle through which the needle tube 12 is turned toward the viewer of FIG. 8(b) is equal to the angle through which the needle tube 12 is turned toward the viewer of FIG. 8(c). In this manner, as shown in FIG. 7, the second ground facet 13b and the third ground facet 13c are symmetrical in shape with respect to the major axis of the edge surface which is of a substantially elliptical shape.
The injection needle 11 having the edge surface 13 of the conventional lancet point structure can easily pierce a skin 7 as the force required to penetrate the skin 7 is small, and poses a reduced burden on the patient. However, inasmuch as the edge surface 13 is of a symmetrical shape, when the needle 11 pierces the skin, or more specifically, when the injection needle 11 pierces a skin 7 perpendicularly thereto for insulin injection by way of self-injection, the sharp needle point 15 on the distal end of the edge surface 13 first makes point-to-point contact with the skin 7, often causing sharp pain. The puncture pain is sustained until the edge surface 13 goes fully through the skin 7. In the present specification, the pain caused when the distal end of the edge surface 13 contacts the skin 7 and the edge surface 13 is further forced into the skin 7 is referred to as “puncture pain”.
In view of the above conventional problems, it is therefore an object of the present invention to provide an injection needle which is capable of reducing puncture pain caused when the injection needle penetrates the skin.